1. Field of the Invention
The present invention relates generally to combustion products smoke alarms and, more particularly, to means for automatically self-testing such alarms.
2. Description of the Prior Art
Smoke alarms, also known as ionization smoke alarms and photoelectric smoke alarms, are extremely effective at reducing deaths from fires. In an effort to maintain this effectiveness over many years, such smoke alarms include a manual test switch. Manufacturers and fire officials recommend that occupants test the smoke alarm's operation periodically, e.g. weekly, by pressing the manual test switch and observing if the smoke alarm produces a perceptible indication that an excessive concentration of combustion products exists about its combustion products sensor, usually by sounding an audible alarm. In addition, battery powered models of smoke alarms also include a battery power monitoring circuit that automatically sounds the audible alarm with a unique sound if a low battery power condition occurs.
The manual test switch included in smoke alarms tests them by electronically simulating the presence of combustion products about the sensor. For example, pressing the manual test switch may electrically connect an impedance in parallel with an ionization chamber included in the smoke alarm. Connecting the impedance in parallel with the ionization chamber changes the voltage thereacross so the electrical signal produced by the ionization chamber simulates that which the chamber produces if an excessive concentration of combustion products are present. Such manual test switches are disclosed in U.S. Pat. Nos. 4,097,850, 4,246,572, and 4,595,914.
In addition to a manual test switch, U.S. Pat. No. 4,595,914 further discloses a smoke alarm that periodically tests whether the sensitivity of the ionization chamber lies within a predetermined range between a minimum and a maximum sensitivity. This patent teaches that the automatic sensitivity test should be performed approximately every minute. Another significant aspect of this patent is that the alarm is inhibited during automatic testing and that it sounds only after the test is completed and only if the ionization chamber's sensitivity is greater than the maximum allowed sensitivity or lower than the minimum allowed sensitivity.
Despite the effectiveness of such smoke alarms at reducing deaths caused by fire, unfortunately, due primarily to dead or missing batteries it is estimated that presently one-fourth to one-third of the installed smoke alarms are not operating. If this trend continues, it has been estimated that by 1994 one-half of all fires in dwellings having alarms will go undetected because the alarms are inoperative. Obviously the preceding situation would not exist if every smoke alarm's operation was tested periodically, e.g. weekly as recommended by their manufacturers and fire officials, and non-functional alarms repaired or replaced. Consequently, the primary cause for this situation is occupant neglect in failing to periodically test the smoke alarm.
There are several reasons why such neglect occurs. First, most smoke alarms are fastened to a ceiling which is typically eight feet above the floor. Consequently pressing the test switch requires either getting a chair or ladder on which to stand while reaching the alarm, or getting a stick with which to push the test switch. This extra effort inclines the owners to neglect testing and may render testing physically impossible for elderly or disabled individuals. Furthermore, since in the absence of a fire there is no readily apparent difference between an operable and an inoperable smoke alarm, occupants forget that they are installed and need to be tested periodically soon after they are installed. For battery powered smoke alarms which produce a sound or signal to indicate a low power condition, another reason for occupant neglect is the rarity of low battery power events. Due to the rarity of low battery power events, some occupants are unfamiliar with the meaning of the sound or signal produced by the smoke alarm when such a condition occurs.